Problem: $f(x) = -x + 5$ for all real numbers. What is $f^{-1}(x)$, the inverse of $f(x)$ ? $2$ $4$ $6$ $8$ $\llap{-}4$ $\llap{-}6$ $\llap{-}8$ $2$ $4$ $6$ $8$ $\llap{-}4$ $\llap{-}6$ $\llap{-}8$
Solution: $y = f(x)$ , so solving for $x$ in terms of $y$ gives $x=f^{-1}(y)$ $f(x) = y = -x+5$ $y-5 = -x$ $-y+5 = x$ $x = -y+5$ So we know: $f^{-1}(y) = -y+5$ Rename $y$ to $x$ $f^{-1}(x) = -x+5$ Notice that $f^{-1}(x)$ is just $f(x)$ reflected across the line $y=x$.